The present invention relates to electronic voltage regulators, and in particular to electronic voltage regulators for use with automobile vehicles, of the type having the output power stage ("pass" stage) formed by a vertical, insulated collector, bipolar PNP transistor.
An electronic voltage regulator is inserted between the generator and the load, formed by the other electronic devices of the vehicle, in order to obtain a continuous supply voltage which is independent of the current absorbed by the load itself.
A regulator must absorb all the electrical stresses reaching its input and must suppress them at its output. It must, in particular, withstand the dangerous positive and negative overvoltages occurring in the electrical circuit of the vehicle, protecting both itself and the load. A strong negative voltage pulse is generated, for example, during the decay transient of the alternator field ("field decay") when the main circuit switch (ignition key) is opened while inductive loads (alternator field winding, ignition, electric motors) are connected to the regulator. An electronic regulator should withstand the high negative overvoltages by behaving as an open switch.
For applications in which a comparatively high output current is required, use is made of an electronic regulator whose output power stage is formed by a bipolar PNP power transistor. This regulator has a high supply efficiency since the relative minimum input-output voltage drop ("drop-out") is equal to the collector-emitter saturation voltage (V.sub.CESAT) of the PNP transistor which is the minimum drop-out which may be obtained in the current state of the art.
The PNP transistor used as the output stage may be a lateral PNP transistor or a vertical, insulated collector, PNP transistor. The latter has a greater current density than the former and has a higher current gain, making its use advantageous since it occupies a smaller area of silicon and supplies a higher current to the regulator output.
However, a vertical, insulated collector, PNP transistor has a reverse breakdown voltage of its base-emitter junction (V.sub.EBO) which is considerably lower than that of a lateral PNP transistor, and as a result thereof, it is unable to withstand high negative overvoltages by behaving as an open switch. These overvoltages in fact cause the breakdown of its base-emitter junction.
In electronic regulators having their power stage formed by a vertical, insulated collector, PNP transistor, in order to prevent the destruction of the transistor in the case of strong negative overvoltages, a protection circuit, which limits these overvoltages, is usually inserted, an embodiment of which is described below. However, this protection circuit occupies a very large area equivalent to the area occupied by the PNP power transistor, as a result of which it is comparatively costly and it is not economically viable to provide both the power components, i.e. the PNP transistor and its protection circuit, and the remaining components of the regulator in a single integrated circuit.